Exhaust aftertreatment systems receive and treat exhaust gas generated from an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
The SCR catalyst is configured to reduce NOx into less harmful emissions, such as N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst. Typically, ammonia is not directly injected into the exhaust gas due to safety considerations associated with the storage of liquid ammonia. Accordingly, conventional systems are designed to inject a reductant (e.g., a urea-water solution) into the exhaust gas, which is capable of decomposing or evaporating into ammonia in the presence of the exhaust gas. SCR systems typically include a urea source and a urea injector or doser coupled to the source and positioned upstream of the SCR catalyst.
Generally, the decomposition of the urea-water solution into gaseous ammonia occupies three stages. First, urea evaporates or mixes with exhaust gas. Second, the temperature of the exhaust causes a phase change in the urea and decomposition of the urea into isocyanic acid (HNCO) and water. Third, the isocyanic acid reacts with water in a hydrolysis process under specific pressure and temperature concentrations to decompose into ammonia and carbon dioxide (CO2). The ammonia is then introduced at the inlet face of the SCR catalyst, flows through the catalyst, and is consumed in the NOx reduction process. Any unconsumed ammonia exiting the SCR system can be reduced to N2 and other less harmful or less noxious components using an ammonia oxidation catalyst.
To sufficiently decompose into ammonia, the injected urea must be given adequate time to complete the three stages. The time given to complete the three stages and decompose urea into ammonia before entering the SCR catalyst is conventionally termed residence time. Exhaust aftertreatment systems typically utilize a long tube of a fixed linear decomposition length that is positioned between the urea injector and SCR catalyst inlet face. The tube usually has at its inlet and outlet a respective curved tube such that exhaust gas flows through a curved section of pipe before entering the linear tube and flows through another curved section of pipe after exiting the linear tube.
Often, the reductant (e.g., urea) is injected into the exhaust aftertreatment system upstream of the inlet of the decomposition tube, and more specifically, at or before the first curved section of pipe coupled to the inlet of the decomposition tube. Due to the high velocity and centrifugal force of the exhaust gas flowing through the first curved section of pipe, the injected liquid reductant is carried toward and sticks to the radially outer portion of the inner wall of the first curved tube. A similar condition occurs at the second curved tube where the high velocity and centrifugal force of the exhaust gas drives the partially decomposed urea into the radially outer portion of the inner wall of the second curved tube, which causes the urea to stick to the inner wall. Further, because the temperature and velocity of the exhaust gas is lowest near the inner wall of the exhaust tubing, the decomposition or evaporation of the urea driven into or near the inner wall of the exhaust tubing due to the curved tubes is significantly reduced. Because less of the urea is decomposed into ammonia, the NOx conversion rate of the SCR catalyst is reduced and the overall efficiency of the SCR system suffers. In addition to lower decomposition rates, curved tube sections tend to reduce the mixing of the urea with the exhaust gas as the urea tends to gravitate together at the inner wall of the bent tubes. Poor urea mixing can lead to a low ammonia vapor uniformity index, which can cause crystallization/polymerization buildup inside the SCR catalyst or other SCR system components, localized aggregation of ammonia, inadequate distribution of the ammonia across the SCR catalyst surface, lower NOx conversion efficiency, and other shortcomings.
Some exhaust aftertreatment systems attempt to improve urea decomposition and mixing by employing a mixer device that spans the entire interior channel of the tubes. The mixers have a plurality of openings and angled blades through which the exhaust and injected urea flow. Although mixers may have certain benefits, they still fall short of adequately facilitating urea decomposition and mixing. Additionally, known mixers are not only bulky and heavy, but are relatively expensive to manufacture and assemble.